Ion implantation is, typically, a physical process that is employed in semiconductor device fabrication to selectively implant dopant into semiconductor and/or wafer material. Thus, the act of implanting does not rely on a chemical interaction between a dopant and semiconductor material. For ion implantation, dopant atoms/molecules are ionized, accelerated, formed into a beam, analyzed, and swept across a wafer, or the wafer is swept through the beam. The dopant ions physically bombard the wafer, enter the surface and come to rest below the surface, at a depth related to their energy.
An ion implantation system is a collection of sophisticated subsystems, each performing a specific action on the dopant ions. Dopant elements, in gas or solid form, are positioned inside an ionization chamber and ionized by a suitable ionization process. In one exemplary process, the chamber is maintained at a low pressure (vacuum). A filament is located within the chamber and is heated to the point where electrons are created from the filament source. The negatively charged electrons are attracted to an oppositely charged anode also within the chamber. During the travel from the filament to the anode, the electrons collide with the dopant source elements (e.g., molecules or atoms) and create a host of positively charged ions from the elements in the molecule.
Generally, other positive ions are created in addition to desired dopant ions. The desired dopant ions are selected from the ions by a process referred to as analyzing, mass analyzing, selection, or ion separation. Selection is accomplished utilizing a mass analyzer that creates a magnetic field through which ions from the ionization chamber travel. The ions leave the ionization chamber at relatively high speeds and are bent into an arc by the magnetic field. The radius of the arc is dictated by the mass of individual ions, speed, and the strength of the magnetic field. An exit of the analyzer permits only one species of ions, the desired dopant ions, to exit the mass analyzer.
An acceleration system is employed to accelerate or decelerate the desired dopant ions to a predetermined momentum (e.g., mass of a dopant ion multiplied by its velocity) to penetrate the wafer surface. For acceleration, the system is generally of a linear design with annular powered electrodes along its axis. As the dopant ions enter therein, they are accelerated there through.
An end station holds one or more target wafers into which an ion beam from the acceleration system implants one or more dopants. The end station is operable to move or scan the one or more target wafers in one or two dimensions as the ion beam strikes the target wafer(s) in order to obtain desired coverage of the target wafer and dose amount in accordance with a prescribed ion implantation process.
One problem that can occur during the ion implantation process is the unwanted introduction of atomic or molecular contaminant particles into the ion beam. These contaminant particles can be introduced into the beam at various stages of the system, such as within the mass analysis subsystem, the acceleration electrodes and/or the end station. These particles can be undesirably implanted into, or deposited onto, one or more target wafers, resulting in degradation or failure of devices formed thereon.